Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The boost pressure may be regulated to a desired set-point through the actions of one or more boost actuators including, for example, a wastegate (WG) coupled across an exhaust turbine and a compressor recirculation valve (CRV) coupled across an intake compressor. The wastegate regulates boost pressure by controlling exhaust gas flow over the exhaust turbine while the compressor recirculation valve is used for compressor surge management. Each boost actuator may be adjusted based on feed-forward and feedback components.
In such boosted engine systems, the presence of high exhaust backpressure can cause one or more exhaust valves of the engine to be held open or forced open when they are commanded to be closed. This unintentional opening of exhaust valves can lead to abnormal combustion events such as misfires, knock, and/or pre-ignition. As such, this can degrade engine performance, as well as reduce the life of various engine components. In addition, engine exhaust emissions may be adversely affected.
One approach for controlling exhaust backpressures in boosted engine systems is shown by Osburn et al. in U.S. Pat. No. 8,621,864. Therein, exhaust gas recirculation (EGR) flow is adjusted to regulate intake air flow based on an existing exhaust pressure. Specifically, intake air flow may be modified by altering EGR flow to regulate exhaust pressure. The EGR flow may be adjusted by one or more of an EGR valve and a variable geometry turbocharger (VGT). The VGT may be controlled to adjust EGR flow based on the existing exhaust pressure and a desired exhaust pressure.
However, the inventors herein have recognized a potential issue with such an approach to controlling exhaust pressures. As one example, adjusting EGR flow may produce changes to intake air flow at a slower than desired rate. To elaborate, intake air flow may respond at a slower rate to variations in EGR flow. As such, higher exhaust backpressures may need to be decreased rapidly in order to reduce abnormal combustion events and their effects on component degradation. Thus, EGR flow adjustments may not avert issues resulting from excessive exhaust backpressures in a prompt manner.
In one example, the issues described above may be at least partly addressed by a method for controlling a boosted engine system comprising: adjusting an intake throttle responsive to pre-turbine exhaust pressure being greater than a threshold without reducing boost level and while maintaining valve timing. In this way, exhaust backpressure in a boosted engine system may be maintained within a range that does not degrade engine hardware and performance.
As one example, a boosted engine system may include a turbocharger and a particulate filter coupled downstream of an exhaust turbine of the turbocharger. A post-turbine exhaust manifold pressure may be measured by a pressure sensor coupled downstream of the turbine. A pre-turbine exhaust pressure may be estimated (that is, predicted or modeled) based on various engine operating conditions including, for example, a load of the particulate filter and/or the post-turbine exhaust pressure. In response to the pre-turbine pressure becoming greater than a threshold, such as a threshold above which the exhaust pressure can hold or force open a cylinder exhaust valve, intake air flow to the engine may be restricted without reducing boost output and while maintaining exhaust valve timing. For example, an intake throttle opening may be reduced to clip the engine intake air flow. A torque control loop of an engine controller may then use the reduced intake airflow as an input to adjust one or more boost actuators to maintain the boost pressure despite the reduced intake air flow.
In this way, excessive exhaust backpressures may be reduced. By adjusting the intake throttle to regulate intake air flow, a faster reduction in intake air charge may be obtained. Thereby, occurrence of abnormal combustion events due to excessive exhaust backpressures may be diminished allowing for improved engine performance and increased durability of engine components. Further, by maintaining boost levels during the lowering of intake air charge, vehicle operator experience may not be degraded. Overall, drivability and emissions compliance may be enhanced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.